1. Field of the Invention
This invention relates to an electrostatic chuck for use in a treatment apparatus such as a plasma etching apparatus to electrostatically hold an object such as a wafer by a coulombic force.
2. Description of the Related Art
In a plasma etching apparatus, for example, an electrostatic chuck is used to electrostatically hold an object such as a wafer.
The electrostatic chuck has an insulating layer and an electrode as basic constituents. An object to be chucked, for example a wafer is placed on the insulating layer. That is, the wafer is aligned with the electrode, with the insulation layer interposed therebetween. At this time, a voltage is applied between the wafer and electrode, thereby causing positive charge on one side and negative charge on the other side. Accordingly, a coulombic force is exerted therebetween, and the wafer is attracted onto the chuck.
For example, an electrostatic chuck is known which is provided with an electrostatic holding sheet having a conductive sheet made of copper and interposed between two insulation polyimide sheets formed on a base member. In this chuck, an object such as a wafer is placed on the holding sheet, and is electrostatically held by the holding sheet when a voltage is applied to the holding sheet by a high voltage power source via a feeding sheet formed integrally with the holding sheet.
This electrostatic sheet, however, will easily be broken since the polyimide insulation layer is formed thin so as to obtain a required coulombic force, and since polyimide itself has a low strength. Further, if the electric charge of the polyimide insulation sheet is removed by using plasma after the wafer is unloaded, the sheet will be damaged by the plasma. It is a great disadvantage of the above-described electrostatic chuck that the life of the polyimide used as an insulating layer is short.
Moreover, the temperature of a wafer is generally controlled via the base of the chuck. That surface of the wafer which is brought into contact with the insulation sheet is small since the wafer surface is uneven microscopically, and the thermal conductivity of polyimide is low. Thus, the thermal conductivity of the chuck is low, which makes it difficult to perform appropriate temperature control of the wafer. Therefore, in the electrostatic chuck, a clearance between the wafer and insulation sheet is filled with a gas under a predetermined pressure to obtain a good thermal conductivity. To obtain a sufficient thermal conductivity without using such a gas, the holding force of the holding sheet must be enhanced. However, it is difficult to do so in the case of the above electrostatic chuck employing an insulating sheet made of polyimide.
To overcome these disadvantages, such an electrostatic chuck has been made which employs an insulating sheet made of a ceramic material such as an alumina sintered body. This chuck is made by using a conventional method for producing a multi-layered ceramic substrate. Specifically, for example, an alumina substrate is formed on a base, and an alumina layer is formed on the substrate with a conductive tungsten pattern interposed therebetween. The electrostatic sheet thus constructed has a ceramic insulator, so that the chuck has a high durability and a high strength against plasma damage, i.e., it is not easily broken. In addition, the wafer-holding force of the chuck can be increased.
However, if electrostatic chucks are produced by mainly using a ceramic sintered body, the yield of the chucks will be low, and the cost thereof will be extremely high, due to distortion and the like caused by sintering.